running-in-ci: skill-edit recipe should use worktree under $TMPDIR (#328)

## Summary

Update the `running-in-ci` "How to propose" recipe so a bot updating
`.claude/skills/` can actually write to disk. The current recipe (`cd
.claude && mv /tmp/... SKILL.md`) satisfies the Claude Code harness
write-guard
([anthropics/claude-code#37157](anthropics/claude-code#37157))
but fails the read-only bind-mount the sandbox places on `.claude/`.
Mirror the pattern `review-runs` already documents: do the edit, commit,
and push from a git worktree under `$TMPDIR`. Both restrictions are now
explained inline so a future reader knows why the worktree is mandatory.

## Why

`worktrunk-bot` hit this in worktrunk run
[24926120125](https://github.com/max-sixty/worktrunk/actions/runs/24926120125)
(a `tend-mention` session triggered by an explicit maintainer
instruction to update repo skill guidance). The bot followed the bundled
recipe, hit `mv: ... Read-only file system`, retried with `cp` (same
error), retried with `touch` (same error), then ad-libbed `git
hash-object -w` plus `git update-index --cacheinfo` to write the blob
and stage it directly to the index — a workaround that left the working
tree showing the file as modified after the fact. ~7 wasted bash cycles
before producing [worktrunk PR
#2415](max-sixty/worktrunk#2415).

`worktrunk-bot` followed the new bundled-skill-defect flow (introduced
in #324 yesterday) and opened permission-request issue
[max-sixty/worktrunk#2416](max-sixty/worktrunk#2416)
on the consumer side. This PR is the corresponding tend-side fix
surfaced by `review-reviewers` rather than waiting on the permission
round-trip — the diagnosis is unambiguous and the fix is small.

## What changes

The "Draft a minimal edit" step (step 3 of "How to propose"):

- Drops the `cd .claude && mv /tmp/... SKILL.md` recipe that fails on
the read-only mount.
- Adds the same prose `review-runs` already carries explaining (a) the
read-only bind-mount of `.claude/` and (b) the harness write-guard from
claude-code#37157, so a reader sees both barriers and why a worktree
clears both.
- Replaces the recipe with the `git worktree add "$TMPDIR/skill-fix"`
pattern, parameterised on `<topic>-$GITHUB_RUN_ID` for the branch name.
- Keeps the existing TODO referencing claude-code#37157.

The frontmatter snippet is moved up so the recipe sits at the bottom of
step 3 with no interleaving.

## Test plan

- [x] Eaten own dogfood: this PR's branch was authored from a worktree
at `$TMPDIR/skill-fix` using exactly the recipe being landed; commit and
push worked end-to-end. Worktree under `$TMPDIR` is writable, no
`Read-only file system` errors.
- [ ] CI on this PR passes (actionlint, ruff, typos, uv-lock).
- [ ] Visual diff vs `review-runs` SKILL.md prose to confirm the two
skills now agree on the recipe shape.

Independent of [#327](#327)
(different concern in the same file: external-tool verification, lines
~498 — no overlap with this edit at lines ~568).

---------

Co-authored-by: continuous-bot <269947486+continuous-bot@users.noreply.github.com>
Co-authored-by: tend-agent <270458913+tend-agent@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: 3 people

plugins/tend-ci-runner/skills/running-in-ci/SKILL.md

@@ -614,28 +614,47 @@ Include in the permission request (and reuse verbatim in the tend issue once app
    ```
    If one is open, add to it instead of opening a second.
 3. **Draft a minimal edit.** One short rule, in the maintainer's words where practical. Place
-   it under an appropriate heading. Use the `Write` tool to author the full new contents of
-   the skill file to `/tmp/running-tend-new.md`, then move it into place via Bash:
-   <!-- TODO(anthropics/claude-code#37157): remove the cd-and-mv workaround once the
-        harness exempts .claude/skills/ as documented — then `Write` directly to the
-        target path. -->
-   ```bash
-   cd .claude && mkdir -p skills/running-tend && cd skills/running-tend \
-     && mv /tmp/running-tend-new.md SKILL.md
-   ```
-   The `cd` is required. Claude Code's harness blocks `Edit`, `Write`, and Bash commands
-   whose write-target argument is a path under `.claude/skills/`
-   ([anthropics/claude-code#37157](https://github.com/anthropics/claude-code/issues/37157)).
-   The guard checks the argument text — `Write(/tmp/…)` and `Bash(mv /tmp/… SKILL.md)`
-   both pass because neither names the protected path.
-
-   New SKILL.md files start with YAML frontmatter:
+   it under an appropriate heading. New SKILL.md files start with YAML frontmatter:
    ```markdown
    ---
    name: running-tend
    description: Project-specific guidance for tend workflows running on this repo.
    ---
    ```
+
+   The checkout's `.claude/` directory is bind-mounted **read-only** under the sandbox
+   (protecting bots from modifying their own skills in place), so edits to `.claude/skills/`
+   files in the working tree fail with `Read-only file system`. Claude Code's harness adds a
+   second restriction on top of the read-only mount: `Edit`, `Write`, and Bash commands with
+   `.claude/skills/` as a write-target argument are denied regardless of filesystem
+   permissions ([anthropics/claude-code#37157](https://github.com/anthropics/claude-code/issues/37157)).
+   The guard checks argument text, so `Write(/tmp/…)` and `Bash(mv /tmp/… SKILL.md)` both
+   pass — the second because `SKILL.md` is a bare filename inside the `cd`'d directory.
+
+   Do the edit, commit, and push from a git worktree under `$TMPDIR`, which is writable and
+   sits outside the harness's `.claude/skills/` write-guard:
+
+   <!-- TODO(anthropics/claude-code#37157): once the harness exempts .claude/skills/ as
+        documented, replace the /tmp-then-mv dance below with direct `Write` to the worktree
+        path. -->
+
+   ```bash
+   git worktree add "$TMPDIR/skill-fix" -b skills/<topic>-$GITHUB_RUN_ID HEAD
+
+   # Use the Write tool to author the new skill file to /tmp/running-tend-new.md.
+   # Then move it into place from inside the worktree. mkdir -p covers the
+   # new-skill case where .claude/skills/<name>/ doesn't yet exist in HEAD:
+   mkdir -p "$TMPDIR/skill-fix/.claude/skills/running-tend"
+   cd "$TMPDIR/skill-fix/.claude/skills/running-tend" && mv /tmp/running-tend-new.md SKILL.md
+
+   cd "$TMPDIR/skill-fix"
+   git add .claude/skills/
+   git commit -m "skills(running-tend): ..."
+   git push -u origin skills/<topic>-$GITHUB_RUN_ID
+   gh pr create --title "..." --body-file /tmp/pr-body.md --head skills/<topic>-$GITHUB_RUN_ID
+   cd -
+   git worktree remove "$TMPDIR/skill-fix" --force
+   ```
 4. **Open as a separate PR.** Follow the repo's PR title conventions (conventional commits,
    Jira prefix, or whatever the repo uses — check recent merged PRs or `CONTRIBUTING.md`).
    The body quotes the triggering feedback and links the thread (PR/issue/comment URL).